Pulse sequencing lateral growth method

ABSTRACT

A process of lateral crystallization is provided for increasing the lateral growth length (LGL). A localized region of the substrate is heated for a short period of time. While the localized region of the substrate is still heated, a silicon film overlying the substrate is irradiated to anneal the silicon film to crystallize a portion of the silicon film in thermal contact with the heated substrate region. A CO 2  laser may be used as a heat source to heat the substrate, while a UV laser or a visible spectrum laser is used to irradiate and crystallize the film.

RELATED APPLICATIONS

This application is a Continuation of a pending patent applicationentitled, PROCESS FOR LONG CRYSTAL LATERAL GROWTH IN SILICON FILMS BY UVAND IR PULSE SEQUENCING, invented by Voutsas et al., Ser. No.10/713,383, filed Nov. 13, 2003.

BACKGROUND OF THE INVENTION

The present invention relates generally to methods of formingpolycrystalline thin films and, more particularly, to methods usinglaser annealing and lateral crystallization.

Polycrystalline silicon thin films are used to form thin-filmtransistors (TFTs) for pixel-switching elements and other integratedcircuits that are simultaneously fabricated on display substrates. Thesethin films and TFTs can provide for the fabrication of integratedcircuits on various substrates, for example, glass, plastic, or metal.These thin films and TFTs may also be used for non-display applicationsas well. Possible non-display applications include, sensors, ASICS,memory modules, or printer heads, for example.

Polycrystalline silicon, also known as poly-Si, films may be produced bycrystallizing amorphous silicon, or microcrystalline silicon. Qualitypoly-Si films can be produced using lateral growth processes, alsoreferred to as lateral crystallization. Quality poly-Si films can thenbe used to produce high performance poly-Si TFTs. The quality of thefilms and the resulting TFTs depends to a great extent on the crystalcharacteristics. Laser induced lateral crystallization, which uses anexcimer laser to crystallize amorphous silicon such that the crystalgrows in a lateral direction, has been used to produce quality poly-Sifilms. By moving the laser and sequentially exposing adjacent regions itis possible to form polycrystalline films having long crystal grainsoriented in the scanning direction.

Despite the success of laser-induced lateral crystallization, andrelated crystallization techniques, in producing quality poly-Simaterials problem areas persist. There is a continued lack of uniformityin the material characteristics of the films produced. The lack ofuniformity results, in part, from the formation of sub-boundaries alongthe direction of lateral growth. The sub-boundaries generate trap stateswithin the active layer, which may modulate device operation resultingin non-uniformity in device characteristics, including thresholdvoltage. Another problem is related to the lateral growth length (LGL),which is the distance that the crystal grows laterally for each lasershot. The LGL is currently limited to between approximately less than3-5 μm. This LGL limitation also affects techniques that employsequential crystallization by scanning, because the moving pitch (p)between sequential laser shots is even more restricted in that themoving pitch should be less than the LGL, (p<LGL). Crystallization overlengths of between about 30 μm and 100 μm, which is desired for devicefabrication, requires many shots, which translates to longer processtimes and reduced productivity.

SUMMARY OF THE INVENTION

Accordingly, a process is provided to increase the LGL. The method mayalso simultaneously increase the spacing between sub-boundaries, therebyreducing the sensitivity of device characteristics to non-uniformityissues caused by sub-boundaries.

The process of lateral crystallization comprises providing a siliconfilm on a substrate surface. A localized substrate region at thesubstrate surface is exposed to a laser heating source. The laserheating source may be a CO₂ laser for example. A portion of the siliconfilm in thermal contact with the localized substrate region iscrystallized by irradiation from a laser annealing source. The laserannealing source may be a UV laser, such as an excimer laser, or avisible laser, such as a frequency-doubled solid state laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a temperature history plot for a silicon film.

FIG. 2 is a temperature history plot for an SiO₂ substrate.

FIG. 3 illustrates a process sequence.

FIG. 4 illustrates a process sequence.

FIG. 5 illustrates a process sequence.

FIG. 6 illustrates a repeating process sequence.

FIG. 7 is a chart of lateral growth length versus substrate temperature.

DETAILED DESCRIPTION OF THE INVENTION

A theoretical study of the physics of laser annealing reveals that thelateral growth length (LGL) is related to the time that the molten Sifilm can remain at, or slightly above, a temperature range thatprohibits nucleation from occurring within undercooled, molten silicon.The temperature range below which nucleation occurs may be referred toherein as the nucleation temperature. The temperature of the Si duringcrystallization depends, in part, on the rate of heat loss from theirradiated domain. The irradiated domain refers to that portion of thesilicon film that is irradiated during laser annealing. Slower heat losswill correspond to a longer time period at, or above, the nucleationtemperature. The longer the time period that the Si within theirradiated domain is above the nucleation temperature, the longer theresulting lateral growth length will be.

Increasing the steady-state temperature of the substrate will reduce therate of heat loss from the cooling molten-Si film. FIG. 1 illustrates atemperature history of laser-irradiated Si films corresponding to threesubstrate temperatures. Temperature history refers to a plot oftemperature versus time at a specific location within the irradiateddomain. FIG. 1 shows plots of the temperature history at the surface ofan irradiated domain of Si film. The first temperature history 12 is fora silicon film over a substrate at room temperature, about 300 K. Thesecond temperature history 14 is for a silicon film over a substrate atabout 900 K. The third temperature history 16 is for a silicon film overa substrate at about 1200 K. Generally, the three plots show some commonpatterns for films irradiated by a laser pulse. The laser pulse heatsthe Si film rapidly increasing its temperature within severalnanoseconds, melting the irradiated region of the Si film. The laserpulse may be between 30 and 300 nanoseconds long, and was 30 nanosecondsin the examples shown in FIG. 1. Once the laser pulse ends, thetemperature of the molten Si film continues to climb, in response to theenergy that has been pumped into the film by the laser pulse until amaximum temperature is reached. The temperature value at the maximumpoint depends on several factors, including the laser fluence, thesubstrate temperature, the pulse duration, and the film thickness, alongwith other factors. Since heat is removed by conduction through thebottom of the irradiated domain, and the surrounding unmelted film, aswell as radiation from the top surface, the temperature of theirradiated domain will reach its maximum value and then decline as heatis removed from the irradiated domain. Conduction tends to dominate theheat removal process. As the temperature in the molten Si region fallsbelow the formal melting point of the film, lateral growth occurs in thefilm. The formal melting point refers to the established melting pointof a bulk material, for example bulk silicon. The molten Si that existsat temperatures below its formal melting point is referred to asundercooled liquid. Such liquid can exist at temperatures substantiallybelow the formal melting point. For example, crystalline silicon has aformal melting point of 1410° C., yet undercooled Si can exist attemperatures as low as approximately 900° C. Lateral growth continues asthe temperature continues to decline. However, the probability ofnucleation is related to the amount of undercooling in the molten-Si.The lower the temperature of an undercooled liquid, the higher theprobability of nucleation occurring within the liquid molten-Si.Nucleation disrupts the lateral growth process and thereby limits thelateral growth length. When nucleation occurs, release of latent heatmomentarily increases the temperature in the recrystallized Si film,which manifests as a hump on the temperature history. Although thenucleation temperature is not fixed, since nucleation is a stochasticprocess of temperature and time, it is possible to identify atemperature band within which the probability for nucleation is highenough to interfere with lateral crystallization processes. This bandhas been empirically determined to be in the range of betweenapproximately 1200 K and 1300 K. The horizontal dashed line in FIG. 1corresponds approximately to the center of this band. After the moltenSi film completely transforms to solid-Si, by pure lateral growth or acombination of lateral growth and nucleation processes, the temperaturecontinues to drop as shown in FIG. 1.

Comparing the three temperature history plots at three differentsubstrate temperatures shows the profound effect that the temperature ofthe substrate has on the characteristics of the temperature history ofthe film. Higher substrate temperatures prolong the average quench time,which is the time the molten-Si remains at temperatures above the bandcorresponding to high nucleation probability. The average quench time 18for the case of the substrate at approximately 1200 K is shown. Thehigher substrate temperature also decreases the solidification velocity,which is the velocity of the advancing solid interface. Thesolidification velocity is given by the slope of the temperature historycurve, which is shown at 22 for the case using an approximately 1200 Ksubstrate temperature, and is related to the propensity of the materialfor defect formation. The higher substrate temperature also increasesthe sub-boundary spacing. Sub-boundary spacing on the order of 2-5 μmcan be obtained as the substrate temperature approaches the Si meltingpoint. In contrast, the sub-boundary spacing is only on the order of0.2-0.5 μm for substrate temperatures corresponding to room temperature.The longer the film is maintained below the formal melting point, butabove the band corresponding to high nucleation probability, the longerthe LGL achieved.

Although the benefits of using a high substrate temperature, for examplegreater than 1200 K, are clear, these temperatures are incompatible withglass substrates commonly used for display substrates. It is impracticalto maintain display glass at steady state temperatures that exceedapproximately 600° C.

However, if the period of heating is sufficiently short and the heatedarea sufficiently small, the high temperature will not cause significantdamage to the substrate while still providing some of the benefits ofhigher substrate temperatures. For example, if the glass substrateimmediately under the irradiated domain of the Si were heated for asufficiently short period of time, the benefits of higher substratetemperature can be obtained, without causing significant substratedamage. Significant damage means substrate damage sufficient to preventfurther processing, or render the substrate wholly unusable for itsintended purpose. A certain amount of degradation of the substrate maytherefore be acceptable, even if it may affect performance parameters orproduction yield.

In one embodiment of the present method, a first laser source is used toirradiate the silicon film, and a second laser source is used to heatthe underlying substrate. Since Si absorbs strongly in the UV—visiblerange, for example below 0.8 μm, the first laser source will preferablyemit in this UV—visible range. Excimer lasers are viable candidates foruse as the first laser source. SiO₂, and glass, such as borosilicateglass, for example CORNING 1737, absorb strongly in the far IR region,from about 9 to 11 μm. The second laser source preferably emits in thisfar IR region. CO₂ lasers are commercially available at wavelengths of10.6 μm, which make them suitable candidates for the second lasersource.

The primary parameters that control the peak temperature and temperaturehistory of a substrate for a single CO₂ laser pulse include, theinstantaneous power (energy/time), the instantaneous power density(instantaneous power/area), and the fluence (instantaneous power densitytimes pulse duration, which correponds to energy/area/pulse). In somelaser systems the instantaneous power is a fixed parameter. In thiscase, the instantaneous power density (IPD) can be adjusted by adjustingthe beam area. Similarly, the fluence can be adjusted by adjusting thebeam area and by controlling the pulse duration. Decreasing the beamarea increases both the IPD and the fluence. However, the fluence can becontrolled independently from the IPD by adjusting the laser pulseduration.

If multiple laser pulses are being used to heat the substrate, theaverage power density may also affect the temperature history. Theaverage power density corresponds to the fluence times the repetitionrate, which also corresponds to the instantaneous power density timesthe duty cycle. The duty cycle is the fraction of time that the CO₂laser is actually firing. The average power density can be controlled byadjusting the beam area, which affects both the fluence and the IPD, byadjusting the pulse width, which affects the fluence, or by adjustingthe repetition rate, which affects the duty cycle and the average powerdensity independent of the fluence or the IPD.

In sequential lateral solidification processes, the annealing lasersource is moved by a scanning distance, which is some fraction of thelateral growth length (LGL) between annealing exposures. If thesubstrate is continuously scanned, the annealing laser source may befired as the desired scanning distance is reached. In an embodiment ofthe present method, the CO₂ laser may be scanned along with theannealing exposure. This results in a given region of the substratebeing exposed multiple times as the scan progresses. Accordingly, thecumulative average power density will affect the temperature history ofthe substrate. The cumulative average power density is the average powerdensity times the number of shots that a given point is exposed to. Inan embodiment of the present method, wherein there is one CO₂ laser shotper annealing laser shot, the number of shots is defined by the distanceacross the beam in the scanning direction divided by the step size ofthe CO₂ laser.

In another embodiment, multiple CO₂ laser shots will be made betweenannealing laser shots as the substrate is continuously scanned. In thiscase, the number of shots is still determined by the distance across thebeam in the scanning direction divided by the step size of the CO₂laser. The CO₂ laser may not have the same step size or distance acrossthe beam as the annealing laser.

In another embodiment, the substrate is scanned in discrete steps andmultiple CO₂ laser shots will be made between annealing laser shots. Inthis case, the number of shots may be determined by multiplying thenumber of CO₂ laser shots between scanning steps times the distanceacross the CO₂ beam in the scanning direction divided by the CO₂ laserstep size.

FIG. 2 shows the temperature history for three pulses, using a 1 mm²beam area for these examples. The pulses are characterized by pulseduration and fluence. The first curve 32 corresponds to a 400 μs pulseat 15 mJ/mm². The second curve 34 corresponds to a 400 μs pulse at 10mJ/mm². The third curve 36 corresponds to a 100 μs pulse at 4 mJ/mm².From these given values for each example pulse, it would be possible tocalculate the IPD. For these examples the time between pulses wasapproximately 3 ms, so the average power density may also be determined.The instantaneous power was not fixed for the purpose of these examples,which illustrate achievable temperatures and the affect of certaincontrol parameters on the temperature histories. The temperaturehistories shown in FIG. 2 do not take into account substrate scanning.

By reviewing a temperature history, similar to those shown in FIG. 2, itwill be possible to approximately select an effective substratetemperature at which the annealing process will occur. According to anembodiment of the present process, the Si film and any supportinglayers, including an SiO₂ substrate, a glass substrate, or an SiO₂ layeroverlying a glass substrate, in contact with the Si film, are irradiatedby two localized irradiation sources. One source is a UV—visible sourcesuitable for crystallizing the Si film. The other source is suitable forheating the SiO₂ substrate. The two sources have a temporal offset,which may be used to select the effective substrate temperature for theirradiated domain for a known peak temperature and temperature history.

In one embodiment, for example, a CO₂ laser is used as a laser heatingsource to heat the SiO₂ layer under the Si film. The Si film isirradiated using either a UV laser, such as an excimer laser orfrequency-tripled solid-state laser, or a visible laser, such as afrequency-doubled solid-state laser, as a laser annealing source. A XeCllaser at 308 nm or a KrF laser at 243 nm are possible candidates for theexcimer laser. Frequency-tripled solid-state lasers, such as tripledNd-YAG lasers or tripled Nd-YVO₄ lasers, are also possible candidatesfor the UV laser Frequency-doubled solid-state lasers, such as doubledNd-YAG lasers operating at 532 nm or doubled Nd-YVO₄ lasers, arepossible candidates for the visible laser.

FIG. 3 schematically illustrates one embodiment of the present process.Two idealized laser pulses are shown. The CO₂ laser pulse 42 starts andheats the SiO₂ substrate. Some time later the excimer laser pulse 44irradiates the Si layer to crystallize it. The time between the start ofthe CO₂ laser pulse and the start of the excimer laser pulse, also knownas the temporal offset (τ_(pre)), can be used to establish the effectivesubstrate temperature at the time of the excimer laser pulse for a giventemperature history. In this example, both the CO₂ laser pulse and theexcimer laser pulse are irradiating the same area at the same time for aperiod of combined irradiation, τ_(comb). During τ_(comb), it ispossible for the molten-Si film to absorb energy from both the excimerlaser and the CO₂ laser. This is because although solid Si ispractically transparent to IR radiation, molten-Si absorbs IR radiation.The total fluence of the two beams may need to be adjusted to avoidagglomeration of the Si film. The CO₂ laser pulse may remain on for aperiod, τ_(post), after the excimer pulse has ended. Some absorptionfrom the CO₂ laser pulse may continue to occur within the molten-Sifilm. This absorption, along with the elevated substrate temperaturereduces the quenching rate in the Si film and increases the effectivetime period for lateral growth, thereby increasing the maximum possiblelateral growth length.

FIG. 4 schematically illustrates another embodiment of the presentprocess. Two idealized laser pulses are shown. The CO₂ laser pulse 42starts and heats the SiO₂ substrate. Some time after the CO₂ laser pulseends the excimer laser pulse 44 irradiates the Si layer to crystallizeit. The time between the start of the CO₂ laser pulse and the start ofthe excimer laser pulse, also known as the temporal offset (τ_(pre)),can be used to establish the effective substrate temperature at the timeof the excimer laser pulse for a given temperature history of thesubstrate based on the CO₂ laser parameters. Since there is no overlapbetween the two laser pulses, the Si film will absorb radiation from theexcimer laser pulse separately from the CO₂ laser. The Si film will beeffectively transparent to the CO₂ laser pulse and so will not absorb asignificant amount of the CO₂ laser pulse. This scheme decouples theeffect of the two different sources, while still providing an effectivesubstrate temperature that will prolong the quenching time. Assumingthat the fastest of the two lasers discharges at a frequency (f), thetemporal offset (τ_(pre)) between the two pulses will be looselyconstrained by the following relations: τ_(CO2)<τ_(pre)<1/f−τ_(fast),where τ_(fast) is the duration of the fast discharge.

FIG. 5 schematically illustrates another embodiment of the presentprocess. Two idealized laser pulses are shown. The CO₂ laser pulse 42starts and heats the SiO₂ substrate. Some time before the CO₂ laserpulse ends the excimer laser pulse 44 irradiates the Si layer tocrystallize it. The time between the start of the CO₂ laser pulse andthe start of the excimer laser pulse, also known as the temporal offset(τ_(pre)), can be used to establish the effective substrate temperatureat the time of the excimer laser pulse by referring to the temperaturehistory of the substrate related to the laser pulse. The excimer laserpulse 44 continues beyond the end of the CO₂ laser pulse. In thisexample, both the CO₂ laser pulse and the excimer laser pulse areirradiating the same area at the same time for a period of combinedirradiation, τ_(comb). During τ_(comb), it is possible for the molten-Sifilm to absorb energy from both the excimer laser and the CO₂ laser.This is because although solid Si is practically transparent to IRradiation, molten-Si absorbs IR radiation. The total fluence of the twobeams may need to be adjusted to avoid agglomeration of the Si film. Theexcimer laser pulse may remain on for a period, τ_(post), after the CO₂laser pulse has ended.

The excimer laser pulse will temporally correspond approximately to themaximum effective substrate temperature if the excimer laser pulse istimed to coincide with the end of the CO₂ laser pulse as shown in FIG.5, or after the end of the CO₂ laser pulse while the temperature of thesubstrate and film continues to increase after the end of the CO₂ laserpulse. In an embodiment of the present method, the temporal offset isdetermined so that the excimer laser pulse corresponds to approximatelythe maximum effective substrate temperature, and the maximum effectivesubstrate temperature is controlled by adjusting the energy introducedinto the substrate, which determines the temperature history. Asdiscussed above, the energy introduced into the substrate by the CO₂laser can be controlled by adjusting a number of factors. Theinstantaneous power could be adjusted in some embodiments. The beam areacould be adjusted to increase or decrease the IPD and the fluence. Thepulse duration could be used to vary the fluence separately from theIPD. The repetition rate, or the corresponding duty cycle, could beadjusted to control the average power density for multiple laser pulses,independent from the factors related to each individual pulse. And inthe case of scanning applications the cumulative average power densityof multiple scanned shots may be futher adjusted by varying the steppingdistance relative to the beam dimension in the scanning direction.

Since the CO₂ laser pulse may be heating a localized region of thesubstrate to a temperature above its formal melting point, it ispreferred to provide a spatial alignment of the laser annealing sourceand the CO₂ laser pulse to maximize the overlap of the two sources. Inthis manner the maximum thermal energy of the CO₂ laser pulse may beused so as to provide the desired benefit without introducing additionalunused heat to the substrate. The formal melting point refers to thesteady state melting point of bulk material, which as discussed hereinmay be exceeded in a localized area for a short period of time withoutcausing significant damage to the substrate.

With any of these pulse sequences, the rate of temperature change in thesubstrate is sufficiently slow relative to the annealing source that thesubstrate has a substantially constant substrate temperature over theapproximately 30 ns-300 ns pulse duration of the annealing source plusthe subsequent approximately 200 ns-500 ns for resolidification. This isapparent by reviewing the difference in the time scale between FIG. 1and FIG. 2. Accordingly, it is possible to establish the effectivesubstrate temperature using a given laser pulse from the laser heatingsource, for example the CO₂ laser, by determining the temporal offset(τ_(pre)), corresponding to the desired substrate temperature based uponthe temperature history of the substrate surface. Similarly, for a giventemporal offset the CO₂ laser pulse sequence may be adjusted to achievethe desired substrate temperature at the time of the annealing laserpulse.

A range of values may be suitable for practicing one or more embodimentsof the present method. The CO₂ pulse area is preferably matched to theexcimer beam area, although larger or smaller beams may be used. CO₂pulse instantaneous power may be selected so as to provide a desiredinstantaneous power density based on the beam area used. The followingranges of values may be used as general guidelines:

CO₂ pulse duration: 5-1000 μs, preferably 5-100 μsec, or 5-30 μsec.

CO₂ pulse area: 1 mm²-1 cm²

CO₂ pulse instantaneous power density: 50-150 W/mm²

CO₂ pulse fluence: 0.4-4 J/cm²; more preferably 0.4-1.5 J/cm²; even morepreferably 0.4-1 J/cm².

Excimer pulse duration: 30-300 ns

Pulse frequency (f): 100-300 Hz.

Temporal offset (τ_(pre)): 5-1500 μs

FIG. 6 shows a repeating pulse sequence corresponding to that shown inFIG. 3 for purposes of a scanning crystallization process. The sequencesshown in FIGS. 4 and 5 could be similarly repeated for use in scanningcrystallization processes.

Lateral growth in Si films using an example of the present process hasbeen found to be substantially longer than without the laser heatingsource. For example, using a CO₂ laser with a pulse duration of 70 μsand a fluence of 0.5-2 J/cm2 and an excimer laser pulse with a temporaloffset of approximately 60 to 70 μs, a lateral growth length of 20 to 25μm can be obtained. The same excimer laser pulse without the use of theCO₂ laser, and a substrate at room temperature, produced a lateralgrowth length of approximately 2 μm. For 50 nm-thick Si films, asub-boundary spacing of 2 μm has been observed, which is a factor ofeight improvement over that obtained without the use of a laser heatingsource.

FIG. 7 is a graph showing the computed dependence of lateral growthlength as a function of effective substrate temperature, using an XeClexcimer laser at 308 nm with a 500 mJ/cm² laser fluence as the laserannealing source, and assuming a single isolated laser heating pulse.Obtaining long lateral growth lengths is a matter of balancing theeffective substrate temperature against the potential damage to thesubstrate as a result of extended exposure to extreme temperatures, evenfor short periods of time within localized regions.

Several embodiments of the present process have been described. Sincemodification of these processes will be apparent to those of ordinaryskill in the art, the following claims shall not be limited to anyspecific embodiment.

1. (canceled)
 2. A process of lateral crystallization comprising:providing a silicon film on a substrate surface; exposing a localizedsubstrate region at the substrate surface to a laser heating source; andannealing a portion of the silicon film in thermal contact with thelocalized substrate region by exposing the silicon film to a laserannealing source.
 3. The process of claim 2, wherein the substratesurface is SiO₂, and the laser heating source has an optical wavelengthof between approximately 9 and 11 μm.
 4. The process of claim 3, whereinthe laser heating source is a CO₂ laser.
 5. The process of claim 4,wherein the CO₂ laser has a pulse duration of between approximately 0.01milliseconds and 1 millisecond.
 6. The process of claim 2, wherein thelaser annealing source is an excimer laser.
 7. The process of claim 6,wherein the excimer laser is a XeCl laser or a KrF laser.
 8. The processof claim 6, wherein the excimer laser has a pulse duration of betweenapproximately 30 nanoseconds and 300 nanoseconds.
 9. The process ofclaim 2, wherein the laser annealing source is a solid-state laser. 10.The process of claim 9, wherein the solid-state laser is afrequency-doubled Nd-YAG laser or a frequency-doubled Nd-YVO₄ laser. 11.The process of claim 9, wherein the solid state-laser is afrequency-tripled Nd-YAG laser or a frequency-tripled Nd-YVO₄ laser. 12.The process of claim 2, wherein the laser annealing source has adischarge frequency of between approximately 100 Hz and 500 Hz.
 13. Theprocess of claim 2, wherein the laser annealing source has a dischargefrequency of between approximately 10 kHz and 100 kHz.
 14. The processof claim 2, wherein the laser heating source is pulsed, the laserannealing source is pulsed, and the laser heating source irradiates thesubstrate prior to irradiation of the silicon film by the laserannealing source pulse.
 15. The process of claim 14, wherein the laserannealing source pulse is shorter than the laser heating source pulse,and starts during the laser heating source pulse.
 16. The process ofclaim 15, wherein the laser annealing source pulse is completed duringthe laser heating source pulse.
 17. The process of claim 14, wherein thelaser annealing source pulse occurs after the laser heating sourcepulse.
 18. A process of lateral crystallization comprising: providing asilicon film in thermal contact with an SiO₂ layer on a substrate;exposing a portion of the SiO₂ layer to a CO₂ laser pulse with aduration of between approximately 0.01 milliseconds and 1 millisecond,whereby the exposed portion of the SiO₂ layer is heated; andcrystallizing a portion of the silicon film in thermal contact with theheated portion of the SiO₂ layer by irradiating the portion of thesilicon film with a pulsed, excimer laser; or a pulsed,frequency-doubled, solid-state laser.